CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statements of Income and Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Revenues
Dividends from subsidiaries										$ 0		$ 0		$ 0
Interest revenue	[1]									70,828		61,579		57,988
Interest revenue—intercompany										0		0		0
Interest expense	[1]									24,266		16,518		12,512
Interest expense—intercompany										0		0		0
Net interest revenue	[1]									46,562		45,061		45,476
Commissions and fees	[1]									11,857		12,707		11,678
Commissions and fees—intercompany										0		0		0
Principal transactions	[1]									9,062		9,475		7,857
Principal transactions—intercompany										0		0		0
Other income										5,373		5,201		5,786
Other income—intercompany										0		0		0
Total non-interest revenues	[1]									26,292		27,383		25,321
Total revenues, net of interest expense		$ 17,124	$ 18,389	$ 18,469	$ 18,872	$ 17,504	$ 18,419	$ 18,155	$ 18,366	72,854	[1]	72,444	[1]	70,797	[1]
Provisions for credit losses and for benefits and claims		1,925	1,974	1,812	1,857	2,073	1,999	1,717	1,662	7,568		7,451		6,982
Operating expenses
Compensation and benefits	[1]									21,154		21,181		20,970
Compensation and benefits—intercompany										0		0		0
Other operating										20,687		21,051		21,368
Other operating—intercompany										0		0		0
Total operating expenses		9,893	10,311	10,712	10,925	10,332	10,417	10,760	10,723	41,841	[1]	42,232	[1]	42,338	[1]
Equity in undistributed income of subsidiaries										0		0		0
Income (loss) from continuing operations before income taxes										23,445		22,761		21,477
Provision for income taxes		1,001	1,471	1,444	1,441	23,864	1,866	1,795	1,863	5,357		29,388		6,444
Income (loss) from continuing operations		4,305	4,633	4,501	4,649	(18,765)	4,137	3,883	4,118	18,088		(6,627)		15,033
Loss from discontinued operations, net of taxes		(8)	(8)	15	(7)	(109)	(5)	21	(18)	(8)		(111)		(58)
Net income (loss) before attribution of noncontrolling interests		4,297	4,625	4,516	4,642	(18,874)	4,132	3,904	4,100	18,080		(6,738)		14,975
Noncontrolling interests		(16)	3	26	22	19	(1)	32	10	35		60		63
Citigroup’s net income (loss)		4,313	4,622	4,490	4,620	(18,893)	4,133	3,872	4,090	18,045		(6,798)		14,912
Comprehensive income
Add: Other comprehensive income (loss)	[2]									(2,499)		(2,791)		(3,022)
Citigroup’s total comprehensive income (loss)										15,546		(9,589)		11,890
Add: Other comprehensive income (loss) attributable to noncontrolling interests										(43)		114		(56)
Noncontrolling interests		$ (16)	$ 3	$ 26	$ 22	$ 19	$ (1)	$ 32	$ 10	35		60		63
Total comprehensive income (loss)										15,538		(9,415)		11,897
Consolidating adjustments
Revenues
Dividends from subsidiaries										(22,854)		(22,499)		(15,570)
Interest revenue										0		0		0
Interest revenue—intercompany										0		0		0
Interest expense										0		0		0
Interest expense—intercompany										0		0		0
Net interest revenue										0		0		0
Commissions and fees										0		0		0
Commissions and fees—intercompany										0		0		0
Principal transactions										0		0		0
Principal transactions—intercompany										0		0		0
Other income										0		0		0
Other income—intercompany										0		0		0
Total non-interest revenues										0		0		0
Total revenues, net of interest expense										(22,854)		(22,499)		(15,570)
Provisions for credit losses and for benefits and claims										0		0		0
Operating expenses
Compensation and benefits										0		0		0
Compensation and benefits—intercompany										0		0		0
Other operating										0		0		0
Other operating—intercompany										0		0		0
Total operating expenses										0		0		0
Equity in undistributed income of subsidiaries										2,163		19,088		(871)
Income (loss) from continuing operations before income taxes										(20,691)		(3,411)		(16,441)
Provision for income taxes										0		0		0
Income (loss) from continuing operations										(20,691)		(3,411)		(16,441)
Loss from discontinued operations, net of taxes										0		0		0
Net income (loss) before attribution of noncontrolling interests										(20,691)		(3,411)		(16,441)
Noncontrolling interests										0		0		0
Citigroup’s net income (loss)										(20,691)		(3,411)		(16,441)
Comprehensive income
Add: Other comprehensive income (loss)										(3,757)		4,277		(2,338)
Citigroup’s total comprehensive income (loss)										(24,448)		866		(18,779)
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0		0		0
Noncontrolling interests										0		0		0
Total comprehensive income (loss)										(24,448)		866		(18,779)
Citigroup parent company | Reportable legal entities
Revenues
Dividends from subsidiaries										22,854		22,499		15,570
Interest revenue										67		1		7
Interest revenue—intercompany										4,933		3,972		3,008
Interest expense										4,783		4,766		4,419
Interest expense—intercompany										1,198		829		209
Net interest revenue										(981)		(1,622)		(1,613)
Commissions and fees										0		0		0
Commissions and fees—intercompany										(2)		(2)		(20)
Principal transactions										(1,310)		1,654		(1,025)
Principal transactions—intercompany										(929)		934		24
Other income										1,373		(2,581)		2,599
Other income—intercompany										(107)		5		(2,095)
Total non-interest revenues										(975)		10		(517)
Total revenues, net of interest expense										20,898		20,887		13,440
Provisions for credit losses and for benefits and claims										0		0		0
Operating expenses
Compensation and benefits										4		(107)		22
Compensation and benefits—intercompany										115		120		36
Other operating										(192)		(318)		482
Other operating—intercompany										49		(35)		217
Total operating expenses										(24)		(340)		757
Equity in undistributed income of subsidiaries										(2,163)		(19,088)		871
Income (loss) from continuing operations before income taxes										18,759		2,139		13,554
Provision for income taxes										714		8,937		(1,358)
Income (loss) from continuing operations										18,045		(6,798)		14,912
Loss from discontinued operations, net of taxes										0		0		0
Net income (loss) before attribution of noncontrolling interests										18,045		(6,798)		14,912
Noncontrolling interests										0		0		0
Citigroup’s net income (loss)										18,045		(6,798)		14,912
Comprehensive income
Add: Other comprehensive income (loss)										(2,499)		(2,791)		(3,022)
Citigroup’s total comprehensive income (loss)										15,546		(9,589)		11,890
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0		0		0
Noncontrolling interests										0		0		0
Total comprehensive income (loss)										15,546		(9,589)		11,890
CGMHI | Reportable legal entities
Revenues
Dividends from subsidiaries										0		0		0
Interest revenue										8,732		5,279		4,590
Interest revenue—intercompany										1,659		1,178		545
Interest expense										5,430		2,340		1,418
Interest expense—intercompany										3,539		2,297		1,659
Net interest revenue										1,422		1,820		2,058
Commissions and fees										5,146		5,366		4,536
Commissions and fees—intercompany										237		182		246
Principal transactions										1,599		1,183		5,718
Principal transactions—intercompany										1,328		1,200		(2,842)
Other income										710		867		191
Other income—intercompany										143		170		306
Total non-interest revenues										9,163		8,968		8,155
Total revenues, net of interest expense										10,585		10,788		10,213
Provisions for credit losses and for benefits and claims										(22)		0		0
Operating expenses
Compensation and benefits										4,484		4,403		4,719
Compensation and benefits—intercompany										0		0		0
Other operating										2,224		2,184		1,983
Other operating—intercompany										2,312		2,231		1,335
Total operating expenses										9,020		8,818		8,037
Equity in undistributed income of subsidiaries										0		0		0
Income (loss) from continuing operations before income taxes										1,587		1,970		2,176
Provision for income taxes										1,123		873		746
Income (loss) from continuing operations										464		1,097		1,430
Loss from discontinued operations, net of taxes										0		0		0
Net income (loss) before attribution of noncontrolling interests										464		1,097		1,430
Noncontrolling interests										0		(1)		(13)
Citigroup’s net income (loss)										464		1,098		1,443
Comprehensive income
Add: Other comprehensive income (loss)										257		(117)		(26)
Citigroup’s total comprehensive income (loss)										721		981		1,417
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0		0		0
Noncontrolling interests										0		(1)		(13)
Total comprehensive income (loss)										721		980		1,404
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Revenues
Dividends from subsidiaries										0		0		0
Interest revenue										62,029		56,299		53,391
Interest revenue—intercompany										(6,592)		(5,150)		(3,553)
Interest expense										14,053		9,412		6,675
Interest expense—intercompany										(4,737)		(3,126)		(1,868)
Net interest revenue										46,121		44,863		45,031
Commissions and fees										6,711		7,341		7,142
Commissions and fees—intercompany										(235)		(180)		(226)
Principal transactions										8,773		6,638		3,164
Principal transactions—intercompany										(399)		(2,134)		2,818
Other income										3,290		6,915		2,996
Other income—intercompany										(36)		(175)		1,789
Total non-interest revenues										18,104		18,405		17,683
Total revenues, net of interest expense										64,225		63,268		62,714
Provisions for credit losses and for benefits and claims										7,590		7,451		6,982
Operating expenses
Compensation and benefits										16,666		16,885		16,229
Compensation and benefits—intercompany										(115)		(120)		(36)
Other operating										18,655		19,185		18,903
Other operating—intercompany										(2,361)		(2,196)		(1,552)
Total operating expenses										32,845		33,754		33,544
Equity in undistributed income of subsidiaries										0		0		0
Income (loss) from continuing operations before income taxes										23,790		22,063		22,188
Provision for income taxes										3,520		19,578		7,056
Income (loss) from continuing operations										20,270		2,485		15,132
Loss from discontinued operations, net of taxes										(8)		(111)		(58)
Net income (loss) before attribution of noncontrolling interests										20,262		2,374		15,074
Noncontrolling interests										35		61		76
Citigroup’s net income (loss)										20,227		2,313		14,998
Comprehensive income
Add: Other comprehensive income (loss)										3,500		(4,160)		2,364
Citigroup’s total comprehensive income (loss)										23,727		(1,847)		17,362
Add: Other comprehensive income (loss) attributable to noncontrolling interests										(43)		114		(56)
Noncontrolling interests										35		61		76
Total comprehensive income (loss)										$ 23,719		$ (1,672)		$ 17,382

[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Notes 1 and 3 to the Consolidated Financial Statements.

[2] Includes the impact of ASU 2018-02, adopted in 2017. See Note 1 to the Consolidated Financial Statements.